前端缓存http请求

需求：
 1、 重复的请求，使用缓存
 2、 不重复的请求，允许发送
 3、 连续两次重复的发送，两次返回的结果是一样的，且第二次不发送请求
 
1、搭建前端服务
vue-cli 一步到位

<template>
  <div class="hello">
    <button v-on:click="getrs(1)">
      北京
    </button>
     <button v-on:click="getrs(2)">
      上海
    </button>
  </div>
</template>
 
<script>
let obj = {};
let flagArr = [];
let objPromise = {};
 
export default {
  name: 'HelloWorld',
  props: {
    msg: String
  },
  methods: {
    getrs(cityId) {
      this.getCity(cityId).then((data) => {
        console.log(data);
      })
    },
    getCity(cityId) {
      let promise = new Promise((resolve) => {
        if(obj[cityId]) {
          /**
           * 如果命中缓存不发送请求了
          */
          return resolve(obj[cityId]);
        }
        
        if(!flagArr.includes(cityId)) {
          flagArr.push(cityId);
          /**
           * 第一次调用的话，请求数据，放到obj里
           */
           this.axios.get('http://localhost:3000/?cityid='+cityId).then((response)=>{
                return response
            }).catch((response)=>{
                return response
            }).then(
                (data) => {
                    let index = flagArr.indexOf(cityId);
                    flagArr.splice(index, 1);
                    obj[cityId] = data;
                    resolve(obj[cityId])
                }
            )
        }
    })
    objPromise[cityId] = promise;
    if(flagArr.includes(cityId)) {
      /**
       * 连续第二次调用的话，如果结果还没有回来，返回上个相同请求的promise
       */
      return objPromise[cityId];
    }
    return promise;
    }
  }
}
</script>
 
<!-- Add "scoped" attribute to limit CSS to this component only -->
<style scoped>
h3 {
  margin: 40px 0 0;
}
ul {
  list-style-type: none;
  padding: 0;
}
li {
  display: inline-block;
  margin: 0 10px;
}
a {
  color: #42b983;
}
</style>

2、后端服务，使用koa，也是一步到位搭建

const Koa = require('koa')
const app = new Koa()
app.use(async (ctx, next)=> {
  ctx.set('Access-Control-Allow-Origin', '*');
  ctx.set('Access-Control-Allow-Headers', 'Content-Type, Content-Length, Authorization, Accept, X-Requested-With , yourHeaderFeild');
  ctx.set('Access-Control-Allow-Methods', 'PUT, POST, GET, DELETE, OPTIONS');
  if (ctx.method == 'OPTIONS') {
    ctx.body = 200; 
  } else {
    await next();
  }
});
app.use(async(ctx)=>{
    let url =ctx.url
    //从request中获取GET请求
    let request =ctx.request
    let req_query = request.query
    let req_querystring = request.querystring
    //从上下文中直接获取
    let ctx_query = ctx.query
    let ctx_querystring = ctx.querystring
    ctx.body={
        url,
    }
})
app.listen(3000,()=>{
    console.log('server is starting at port 3000');
})
const Koa = require('koa')
const app = new Koa()
app.use(async (ctx, next)=> {
  ctx.set('Access-Control-Allow-Origin', '*');
  ctx.set('Access-Control-Allow-Headers', 'Content-Type, Content-Length, Authorization, Accept, X-Requested-With , yourHeaderFeild');
  ctx.set('Access-Control-Allow-Methods', 'PUT, POST, GET, DELETE, OPTIONS');
  if (ctx.method == 'OPTIONS') {
    ctx.body = 200; 
  } else {
    await next();
  }
});
app.use(async(ctx)=>{
    let url =ctx.url
    //从request中获取GET请求
    let request =ctx.request
    let req_query = request.query
    let req_querystring = request.querystring
    //从上下文中直接获取
    let ctx_query = ctx.query
    let ctx_querystring = ctx.querystring
    ctx.body={
        url,
    }
})
app.listen(3000,()=>{
    console.log('server is starting at port 3000');
})

3、测试结果
实现点击多次北京
一次请求，但是执行结果是一样的
来回点击北京上海
一共两次请求，北京的得到是北京的请求结果，上海得到是上海的请求结果